The compilation command for PLATON should look like:

(sudo) gfortran -o platon platon.f xdrvr.c -L/usr/X11/lib -lX11 -I/usr/X11/include

Sometimes, 'usr' should should be 'opt' in the above command. 

sudo appears to be needed for compilation on Mac OS X/Yosemite but not
with linux distributions.

Mac OS X Yosemite:

Install the latest XQuartz, gfortran, gcc, Xcode and command line tools